A biocide is any substance that kills microorganisms such as bacteria, molds, algae, fungi or viruses. A biostatic is any substance that inhibits the growth of these organisms. The collective group is called antimicrobials. People have been utilizing antimicrobials, commonly called preservatives, since they first discovered a need to extend the useful life of their food as well as their possessions. Sea salt may have been the first antimicrobial used to preserve food. The mummification techniques employed by early Egyptians used to preserve the human and animal body used salts and a variety of resins. These preservatives were thought to possess magical powers, as well as the ability to install qualities of eternal life.
The existence of microorganisms in nature was discovered in the late 1600s with the invention of the microscope. As early as 1705, mercuric chloride was used to preserve ships' planking against shipworm. It was not until the 19th century discoveries by Pasteur, Gram and others that the causative agents of microbiological deterioration were understood, although use of antimicrobials in a cause and effect relationship with microorganisms is less than a century old.
It is well known in the art that certain slanol quaternary ammonium compounds possess bacteriostatic, fungistatic and algaestatic and/or bactericidal, fungicidal and algaecidal properties. See, for example, U.S. Pat. Nos. 3,730,701; 3,817,739; and 4,394,378; and British Patent No. 1,386,876. For example, one such compound, 3-(trimethoxysilyl)propyl octadecyldimethyl ammonium chloride is a commercial antimicrobial product marketed by Dow Corning as “Bioguard Q 9-5700” (EPA No. 34292-1). U.S. Pat. No. 3,794,736 describes a number of other organoslicon amines and salts thereof exhibiting antimicrobial activity on a wide variety of microorganisms.
This technology utilizes the properties of reactive silanols and their ability to bond with a target surface. The reactive silanol will form a covalent bond with any surface containing oxygen, nitrogen or carbon in any form. For example hydroxides or oxides on the surfaces of metals (including stainless steel) will form a durable bond. In addition, silanol groups will homopolymerize via a condensation mechanism to form a durable, 3 dimensional crosslinked polymer matrix. The application is therefore very versatile and many types of surfaces may be treated, such as plastic, metal, fabric, tile, masonry, vinyl, wood, painted surfaces and human and animal skin, hair and nails.
The silanols are modified with biocidal adjuncts in the form of alkyl quaternary ammonium groups, so that when the silanols fix onto a surface, the active biocidal sites become fixed too. The films created are extremely thin, between 15 nm and 180 nm, and therefore the original physical properties of the surface are little affected.
Bacteria arriving on the surface encounter the hydrocarbon portion of the biocidal adjunct that may be assimilated into the cell without any disruption. However, contact with the positively charged nitrogen atom will unbalance the electrical equilibrium within the porin channels and on the outer protein layers such that the cells can no longer function correctly and the microbes will die without electron transfer. Therefore the positively charged nitrogen is immediately prepared to subsequently kill additional microbes. Since the kill is electrical and not poison, SQACs do not produce new, resistant strains of microbes such as MRSA.
The fixed nature of the SQAC biocide is important where toxicity, taint and other organoleptic aspects are of concern. This bactericidal surface treatment is not removed by normal cleaning procedures. In fact, it is important to maintain the normal cleaning regime in order to ‘refresh’ the biocidal surface. The thinness of the film enables application in areas where optical properties are important such as treatment of contact lenses. The technology has been used for treatment of bed sheets, hospital garments (Murray et al, 1988), curtains, floor and wall materials, air filtration systems, medical devices, bandages, surgical instruments and implants (Gottenbos et al, 2002). The technique has been used to prevent biofilm growth on catheters, stints, contact lenses and endotracheal tubes.
Hospital Acquired Infections are responsible for 100,000+ deaths per year in the United States alone. The SQAC technology is used to treat human skin where the SQAC covalently bonds to form a durable, antimicrobial barrier that lasts through many washes and provides up to 3 days of reduction or elimination of bacteria, viruses and fungi including the following non-limiting specific list of microbes (Peterson et al, 2003):
Bacteria:
Gram Positive Bacteria:
    Citrobacter freundii     Citrobacter diversus     Corynebacterium diptheriae     Diplococcus pneumoniae     Micrococcus sp. (I)    Micrococcus sp. (II)    Micrococcus sp. (III)    Mycobacterium spp.    Staphylococcus albus     Staphylococcus aureus     Staphylococcus citrens     Staphylococcus epidermidis     Streptococcus faecalis     Streptococcus pyogenes Gram Negative Bacteria:    Acinetobacter calcoaceticus     Enterobacter aerogenes     Enterobacter aglomerans (I)    Enterobacter aglomerans (II)    Escherichia coli     Klebsiella pneumoniae     Nisseria gonorrhoeae     Proteus mirabilis     Proteus morganii     Proteus vulgaris     Providencia spp.    Pseudomonas     Pseudomonas aeruginosa     Pseudomonas fragi     Salmonella choleraesuis     Salmonella enteritidis     Salmonella gallinarum     Salmonella paratyphi A    Salmonella schottmuelleri     Salmonella typhimurium     Salmonella typhosa     Serratia marcescens     Shigella flexnerie Type II    Shigella sonnei     Virbrio cholerae Viruses:Adenovirus Type IV    Feline Pneumonitis     Herpes Simplex Type I & II    HIV-1 (AIDS)    Influenza A (Japan)    Influenza A2 (Aichi)    Influenza A2 (Hong Kong)    Parinfluenza (Sendai)    Poliovirus    Reovirus    Respiratory SynctiaFungi and Mold:    Alternaria alternate     Asperigillus niger     Aureobasidium pullulans     Candida albicans     Cladosporium cladosporioides     Drechslera australiensis     Gliomastix cerealis     Microsporum audouinii     Monilia grisea     Phoma fimeti     Pithomyces chartarum     Scolecobasidium humicola     Trychophyton interdigitale     Trychophyton mentagrophytes 
The EPA's 2007 toxicity ruling on SQAC in the same EPA's Pesticide Docket #EPA-HQ-OPP-2007-0831 states “Upon reviewing the available toxicity information, the Agency has concluded that there are no endpoints of concern for repeated oral or dermal exposure to the trimethoxysilyl quats. This conclusion is based on low toxicity observed in acute, subchronic and developmental studies conducted with the trimethoxysilyl quat compounds. There are no concerns for carcinogenicity for the trimethoxysyl quats based on the results of the mutagenicity studies and the lack of any systemic toxicity being observed in the toxicity data base; therefore, no carcinogenic analysis is required.”
The EPA's 2007 environmental fate ruling on SQAC as stated in EPA's Pesticide Docket # EPA-HQ-OPP-2007-0831 states “The Agency has conducted an environmental fate assessment dated Sep. 19, 2007 for the trimethoxysilyl quats. The hydrolysis data indicate that the trimethoxyslyl quats are soluble but not stable in water. Environmental fate studies for the trimethoxyslyl quats consist of only a hydrolysis study and it was concluded by the Agency that no further fate studies would be required because of the instability of the compounds and their formation of an insoluble silane degradate. The trimethoxysilyl quats are not expected to contaminate surface or ground water due to rapid degradation by hydrolysis.”
For health, safety and economical reasons, it is most desirable to apply such antimicrobial SQACs from an aqueous medium, which may contain additives and components the purpose of which is to increase long term aqueous storage stability against homopolymerization of the hydrolyzed silanol groups causing viscosity increase and/or precipitation, provide scent and maintain solution clarity, improve performance and protect against aqueous mold growth.
Imparting long-term storage stablity, however, to the newly formed aqueous SQAC solution, is a major concern that directly impacts the use and marketability of such antimicrobial formulations. Experience has shown that even low aqueous concentrations are unstable, leading to the premature sedimentation of polysilsesquioxane-type polymers or to rapid increases in solution viscosity causing disruption to standard methods of coating applications. To improve shelf-life and storage stability, thus yielding a marketable formulation, many strategies have been implemented.